1. Field of the Invention
The present invention relates to an attenuator, and more particularly relates to an attenuator having a circuit element that adjusts the attenuation characteristic of the attenuator.
2. Description of the Related Art
An attenuator has been known as a circuit having a function to attenuate a gain of an input signal, and is used for a cellular phone, for example. In the cellular phone, for example, an attenuator is provided between an antenna that receives a signal and a low noise amplifier that adjusts a gain of the received signal. In this case, the attenuator has a role to attenuate the gain of the input signal so that the gain of the received signal may not exceed the dynamic range of the low noise amplifier. Recently, it has been required to perform communications by using a high frequency signal in wideband such as ultra wideband communications systems. Thus, an attenuator capable of handling wideband high-frequency signals as been required.
However, in some cases, the attenuation characteristic of an attenuator may be sharply changed depending on the frequency of an input signal under the influence of a parasitic element component of a circuit element which constitutes the attenuator. This is because the impedance of the parasitic element component changes relative to the frequency of the input signal. As the frequency of the input signal increases, a change in the attenuation characteristic of the attenuator becomes large. If the amount of attenuation of the gain of the input signal changes greatly depending on the frequency of the input signal, the low noise amplifier that receives an output signal from the attenuator has to have a complicated circuit configuration such that the amplifier can correspond to the change in the gain of the output signal of the attenuator. Thus, it is important to adjust the attenuation characteristic of the attenuator and to design the attenuator capable of handling wideband high-frequency signals.
There are mainly two kinds of attenuator configurations. One is a Π-type attenuator and the other is a T-type attenuator. In the Π-type attenuator, circuit elements are connected in the form of a Π-type circuit in a two terminal pair network. Meanwhile, in the T-type attenuator, circuit elements are connected in the form of a T-type circuit in a two terminal pair network. Japanese Patent Application Publication No. 2000-286659 (Patent Document 1) discloses a technique related to an attenuator in which the Π-type attenuator is combined with the additional T-type attenuator. FIG. 11 shows the attenuator described in Patent Document 1. This attenuator adjusts the amount of attenuation of an input signal by adjusting a value of a control voltage applied to a control terminal 1008 and a value of a bias voltage applied to a bias terminal 1021. For example, Patent Document 1 describes an attenuation technique in which while the bias voltage is applied to the bias terminal 1021 to drive PIN diodes 1004, 1006, 1010, 1012, and 1014, the value of the control voltage applied to the control terminal 1008 is increased. Thereby, internal resistances in the PIN diodes 1010, 1012, and 1014 are decreased, while internal resistances in the PIN diodes 1004 and 1006 are increased, so that the amount of attenuation of the gain of the input signal received at an input terminal is increased. In other words, in the technique described in Patent Document 1, the attenuation characteristic of an attenuator 1000 is adjusted by adjusting the balance between the control voltage to be applied to the control terminal 1008 and the bias voltage to be applied to the bias terminal 1021.
The present inventor has found that the conventional technique according to Patent Document 1 has the following problem. As described above, it is important to adjust the attenuation characteristic of an attenuator, and to design the attenuator capable of handling wideband high-frequency signals. However, in the technique described in Patent Document 1, the attenuation characteristic of the attenuator is adjusted by the values of the voltages to be applied to the control terminal and the bias terminal. In this case, for example, a step-down circuit for adjusting the value of the voltage to be applied to the bias terminal is required, so that the circuit scale of the attenuator is increased. In addition, since the attenuation characteristic of the attenuator is adjusted by the voltage to be applied to the terminal in Patent Document 1, a thermal noise and a shot noise may in some cases be mixed in an output signal of the attenuator, the thermal noise and the shot noise being turbulence of a voltage signal caused by the random motion of electric charges to be superimposed on the applied voltage. Since a receiving circuit of communications equipment is not to handle a signal having a high gain, the noise component has a great influence on a signal. For this reason, it is required that the receiving circuit have a circuit configuration of not generating a noise as much as possible.